\begin{ganttchart}[
	vgrid={*3{dotted},*1{dashed},*7{dotted},*1{dashed},*3{dotted},*1{dashed},*3{dotted}},
	hgrid,
	y unit title=0.75cm,
	title label anchor/.style={below=-1.5ex}]{20}
	\gantttitle{Major frame}{20} \\
	\gantttitle{Minor 1}{4}
	\gantttitle{Minor 2}{8}
	\gantttitle{Minor 3}{4}
	\gantttitle{Minor 4}{4} \\
	\ganttbar[bar/.append style={fill=CornflowerBlue}]{Subject 1}{1}{4}
	\ganttbar[bar/.append style={fill=CornflowerBlue}]{}{13}{16} \\
	\ganttbar[bar/.append style={fill=YellowGreen}]{Subject 2}{5}{12}
	\ganttbar[bar/.append style={fill=YellowGreen}]{}{17}{20}
\end{ganttchart}
